Consider: it is noted specifically in the Army books and wiki that (bolded emphasis mine):
- were enchanted with Dark Magic that would both corrupt and enhance the unborn Dragons within.
- Due to the excess of Dark Magic pulsing within their bodies, Black Dragons are notoriously stronger than their pure-blooded counterparts, without the horrific side-effects of mutation common in most such tainted creatures.
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So, really, consider that between two dragons, one of Ulthuan or even just the Old World, compared to a Naggarothi Black Dragon, on average it is the latter that is stronger.
It also depends on how you consider maturity speeds in dragons. If, going by Asur nomenclature, a Sun Dragon is young and fiery, but then:
Those Dragons that surpass the Sun Dragons in might and enlightenment are known as Moon Dragons
That doesn't actually tell us ages. It just tells us strength and presumably a nebulous mind/wisdom/etc. thing which encompasses their standard of enlightenment. Only the Star Dragons are given even a vague age bracket i.e. poetically 'as ancient as the very stars of firmament'.
So is it not possible for a newborn dragon to be born on Ulthuan that is exceptionally strong and remarkably wise? Much like how we can have people whose minds and bodies seem to outdo their peers in real life despite age similarities? Do you call that a Moon Dragon, then?
So really, age has something to do with it, yes, but so does flat out might and mind count as well.
As far as I know, the Black Dragons are not termed by the Caledorian Sun/Moon/Star, nor by the Old World Dragon/Great Dragon/Emperor Dragon rankings.
Just...Black Dragons. Malekith's mount is noted to have been directly stolen and warped for a century from Caledor itself when the Druchii first fled to Naggaroth and was such a prick that when it was born it began destroying all the eggs around it that it could immediately. It's likely benefitted from regular 'enhancing' efforts as well since to keep up as they continue to experiment. But Seraphon is likely the oldest Black Dragon alive now. Is it a Star Dragon equivalent now, or was it already centuries or longer ago?
They've also presumably been forcing production as they like as well, because there are approximately 0% signs that there is any lethargy affecting the Black Dragons, and by 8th Edition powerful Druchii can afford to simply purchase eggs to devour to give themselves considerable power ups. Which no sane Asur would even consider with their equivalents which are noted to be quite few in their egg laying and hatching.